Vehicle trains, particularly tractor-trailer combinations for use on roads, employ brakes both in the tractor and in the trailer to improve the ability to stop the vehicle train. For ordinary stops, the brakes are applied gradually; and the tractor brakes and trailer brakes take effect almost simultaneously.
However, in emergencies when the brakes are applied rapidly, the high pressure air passing through the air control line will reach the tractor brakes before it reaches the trailer brakes. Accordingly, the tractor brakes are applied first, and the trailer tends to push the tractor in the brief interval before its brakes are applied. When such a situation occurs, if the tractor and trailer are not almost perfectly aligned, the trailer will push the tractor off to the side, bending the connection between the tractor and the trailer sharply to produce a dangerous and damaging situation known in the trucking business as jackknifing of the rig.
In the past, jackknifing has been avoided by brake arrangements where trailer brakes are applied first, but such arrangements are unsatisfactory for other reasons, including undue strain on the coupling between the tractor and the trailer.